Hepatic lipase (HL) 1 and lipoprotein lipase (LPL) are members of the same lipase gene family, along with pancreatic lipase, the pancreatic lipase-related lipases, endothelial lipase, and phosphatidylserine-specific phospholipase A 1 (1-6). Through their ability to hydrolyze triglycerides and phospholipids in a variety of circulating plasma lipoproteins, including chylomicrons and very low, intermediate, and high density lipoproteins, HL and LPL greatly influence lipid metabolism (7-9). HL and LPL are associated with cell surfaces through an interaction with heparan sulfate proteoglycans and are thought to possess non-catalytic functions associated with the binding and clearance of various lipoproteins (10 -13). HL and LPL share a number of functional domains such as the SerAsp-His catalytic triad, heparin-binding domain, lid region, and lipid-and receptor-binding domains (15). Based on their similarity of lipolytic function, amino acid homology, and conservation of disulfide bridges, it is believed that HL and LPL share a similar structure (16). Despite these similarities, however, differences remain in important enzyme characteristics such as relative heparin affinity, substrate specificity, and cofactor requirements.Unlike HL, LPL requires a specific cofactor, apoC-II, to hydrolyze triglycerides in chylomicrons (17,18). The importance of apoC-II for LPL function is emphasized by the observation of a significant accumulation of triglycerides in patients who have an inherited defect of the apoC-II gene (19). Initially, the study of chimeric lipases (20,21) suggested that a region in the N-terminal domain of LPL was responsible for cofactor activation because enzymes containing the N-terminal domain of LPL and the C-terminal domain of HL were still able to be activated by apoC-II. However, these chimeric enzymes were not activated by apoC-II to the same extent as native LPL. More recently, we reported that the 60 C-terminal amino acids of LPL also participate in apoC-II activation (22), suggesting that regions in the N-terminal domain alone are not sufficient to achieve optimal activation. These results are more easily interpreted in the context of a head-to-tail dimer model (15,20,(23)(24)(25), which supports the hypothesis that apoC-II interacts simultaneously with regions located in the N-and C-terminal domains of opposing subunits that make up an LPL dimer (22).To identify specific LPL amino acid residues that are responsive to cofactor, chemical cross-linking of apoC-II to LPL was undertaken. Cross-linking experiments identified a region from the N-terminal domain of LPL that interacted with